People that do full-volume boils start with more because evaporation will cause the final volume to be less. Some employ partial-boils, but here are a few reasons to do full-boils: better hop extraction, lighter-colored wort, and entire volume of wort is sterilized by the boil. Each system has a different evaporation rate, so some people may start with 6 gals, others may start with 7 gals to end up at 5 gals.

Hops can be used in the wort 3 ways: bittering, flavoring, and aromatics. The longer hops stay in the boil, the more their bittering acids are dissolved into the wort (up until a point). Hop additions with 10-20 minutes remaining add flavor because the boil isn't long enough to totally isomerize the bittering acids. Lastly hop additions with a few minutes remaining, or using a hopback, have the greatest effect on aroma as you're allowing the hop's essential oils responsible for aroma pass through.